Radiofrequency (RF) signals have use in household radios, radio telecommunication networks, microwave telecommunication networks, and many other systems. Radiofrequency signals are characterized by a frequency phase. Microstrip transmission lines carry radiofrequency signals between devices within a circuit. Manufacturing microstrip transmission lines usually includes forming a thin layer of conductive material, e.g., copper, on the top nonconductive dielectric surface of a circuit board. Microstrip transmission lines form the necessary connections between the components on an RF circuit board.
In certain applications, adjusting the phase or phase tuning an RF signal is necessary. Altering the impedance of the microstrip transmission line on which the signals are travelling is one method of phase tuning. For example, for a signal travelling at a 94 degree phase on a microstrip transmission line relative to a reference, by reducing the impedance of the microstrip transmission line the phase of the signal can be changed to 90 degrees relative to the reference.
The impedance of a microstrip transmission line results from the inductance and the capacitance of the material used to make the microstrip transmission line. The material, amount of material, and shape of the material used to form the microstrip transmission line affect the impedance of a microstrip transmission line. Different types of materials, for example, copper versus silver, have associated different impedance characteristics. Also, long, thin microstrip transmission lines have higher impedance, and inversely, short, wide microstrip transmission lines have lower impedance. The shape of the microstrip transmission line also adds to the impedance of the line. Straight microstrip transmission lines will, for example, have less impedance than spiral microstrip transmission lines.
Past approaches to alter the impedance of a microstrip transmission line include using RF variable capacitors, tuning chips, and capacitive stubs. Using RF variable capacitors requires attaching the capacitor to the microstrip transmission line. Signals are phase tuned by adjusting the value of the variable capacitor. The disadvantages of RF variable capacitors include their high cost, large size, and construction which is subject to mechanical failure and microphonics. Additionally, the packaging of RF variable capacitors provides a minimum capacitance which cannot be adjusted out, and also, affects the capacitance characteristic of the RF variable capacitor in applications above 2 gigahertz.
A second approach for altering the impedance of a microstrip transmission line is by attaching tuning capacitive chips of metal or dielectric to the microstrip transmission line with an adhesive. Using tuning capacitance chips has many disadvantages including the time and labor necessary to determine the proper size and location of the chip, and attaching the chip to the microstrip transmission line. Tuning chips can also fall off the circuit board if the adhesive fails.
A third approach for altering the impedance of a microstrip transmission line is using capacitive stubs connected to the microstrip transmission line. In this approach, a length and width of material of the same type as the microstrip transmission line are made a part of the microstrip transmission line. The capacitive stub provides impedance to the microstrip transmission line. Removing a portion of the length of the capacitive stub alters the impedance of the microstrip transmission line. The change in impedance thereby alters the phase of a radiofrequency signal travelling on the microstrip transmission line.
Using capacitive stubs is a very reiterative process. This process involves measuring the phase of the signal relative to a reference, removing portions of the capacitive stub and then remeasuring the phase of the signal. These steps are duplicated until the desired phase shift in the signal is achieved. If too much of the capacitive stub is removed during the phase tuning of the RF signal then the circuit board is ruined. Once portions of the capacitive stub are removed, the capacitive stub cannot be rebuilt. Since it is impossible to determine beforehand how much of the capacitive stub to remove in order to phase tune the RF signal, this process often results in unusable circuit boards.
Unfortunately, this result is exasperated because tuning the signal with a capacitive stub often occurs only after the circuit board has been populated with other devices. Therefore, if the signal cannot be adequately tuned by altering the capacitive stub, then the components on the circuit board have to be removed for placement on another circuit board. Removing the components from the circuit board often results in damage to these sometimes expensive parts, and leads to not only scrapping the original circuit board, but also all of the components that were previously placed on the circuit board.
It is an object of the present invention, therefore, to provide an improved method and apparatus for altering the impedance of a microstrip transmission line that overcomes the limitations of prior art capacitive devices. It is an object of the present invention to provide a interdigital capacitor that is connected to the microstrip transmission line. The technical advantage of the present invention is the interdigital capacitor is in a shape such that, by removing portions of the interdigital capacitor, a predetermined and known change to the capacitance of the interdigital capacitor and, therefore, the to impedance of the microstrip transmission line is achieved. This in turn provides a predetermined and known result in the phase of the RF signal travelling on the microstrip transmission line. It is a further object of the present invention to provide an apparatus and method using the interdigital capacitor coupled to the microstrip transmission line to phase tune a radiofrequency signal travelling on the microstrip transmission line. By measuring the phase of a signal relative to a reference and knowing the predetermined impact to the phase of the signal by removing a known quantity of impedance from the interdigital capacitor coupled to the microstrip transmission line, the phase of the signal can be altered without the reiterative processes or limitations of the prior art.